The invention generally relates to power control systems for signal transmission systems, and relates in particular to power monitoring systems for radio frequency (RF) transmitters.
Conventional transmitters for wireless telephone cellular networks generally require strict control of transmitted RF power. In wireless cellular networks, strict power control permits the size of cells to be precisely set to enhance coverage. Precise power control also avoids the need for excessive thermal dimensioning of the RF power amplifier (PA), which is required when there is uncertainty about the actual transmitted power. For example if a 50 W (47 dBm) power amplifier has a transmit power variability of even ±1 dB, the PA must be dimensioned so that it may safely (i.e., without overheating) transmit 63 Watts (48 dBm). Power measurement and control is also used in the receive section, usually at intermediate frequencies (IFs). In certain applications it is desirable to measure and control the gain of the received signal so that IF amplifiers and analog-to-digital converters (ADCs) are not overdriven. While precision in the measurement of the received signal (commonly referred to as received signal strength indicator or RSSI) is useful for maximizing the signal-to-noise ratio, it is less important than on the transmit side, the goal being to merely keep the received signal under a certain limit.
Root-mean-square (RMS) RF power detectors are capable of measuring RF power independent of signal peak-to-average ratio or crest factor. This is critical when the peak-to-average ratio of a measured signal is changing, which is common in wireless cellular networks due to the ever-changing number of calls being carried by a cellular basestation. The changing peak-to-average ratio results both from the transmission of multiple carriers at varying power levels and from the variations in code-domain power in a single Code Division Multiple Access carrier.
RMS power detection, therefore, is required to measure and control transmitted power in multi-carrier wireless infrastructure. Traditional power detectors, using diode detection or log amps do not accurately measure power when the peak-to-average ratio of the transmitted signal is not fixed.
There is a need, therefore, for a system and method for efficiently and accurately measuring the power in a multi-carrier wireless infrastructure.